The Friends List has four tabs. The first displays your friends playing on the same platform, and the second includes your buddies playing on different systems. The other tabs are for recent players and notifications, including those all-important party invites. You'll also see invites for clubs, which now also work across platforms.
To add a friend, you'll need to know their RocketID, which combines their username with a four-digit number. This is automatically assigned, though you can change your RocketID username from an option at the bottom of the list (the number will stay the same).
Psyonix said in October it was delaying its RocketID system until this year, to make sure it was compatible with PS4. After Sony gave the thumbs up for cross-platform play, Rocket League players now only need to wait one more week until they can form elite car soccer teams with their pals on other platforms.
Meanwhile, next week's update also brings practice options for extra modes (including Hoops and Rumble), Competitive Season 9 rewards and a way to see your number of weekly wins. However, an event that was planned for this month has been canceled. Psyonix will focus instead on a different upcoming event.